Tracking the ultrafast Mott-crossover of electron-hole pairs of excitonic states

Mott-crossover is a fundamental quantum transition emerging in various many-body systems associated with strong correlation effects. The fundamental understanding of one of the first known Mott-crossover, the association and dissociation of Coulomb correlated electron-hole (e-h) pairs in photoexcited semiconductors, is still elusive because of the lack of direct access to the single-particle band structure of the transition. Here, we report the direct momentum space visualization of time-varying e-h many-body states and correlations via extreme-ultraviolet time- and angle-resolved photoelectron spectroscopy. The time evolution of photoemission signatures from a monolayer semiconducting MoS2 captures the picosecond crossover from unbound carriers to excitons, manifested by the appearance of a new spectral feature with evolving energy-momentum relation across the Mott-crossover. Excitation-energy-dependent studies around the conduction-band threshold, along with theoretical calculations, further substantiate these observations. Our work highlights the application of ultrafast photoelectron probes to access fingerprints of time-varying e-h correlations, thus opening up new paths in the study of many-body quasiparticle phases and transitions across energy, momentum, and time. Mott-crossover, a key quantum transition in strongly correlated systems, remains poorly understood due to challenges in accessing single-particle band structures. Here, the authors use extreme-ultraviolet time- and angle-resolved photoelectron spectroscopy to visualize the dynamic evolution of electron-hole pairs in MoS2, providing insights into many-body quasiparticle phases and transitions.